fix(client): treat empty-string env credentials as unset

[Zawwarsami16]

Summary

Closes #1640.

When ANTHROPIC_API_KEY / ANTHROPIC_AUTH_TOKEN are present in the environment but set to an empty string, the client treats the empty value as a real credential rather than as absent. It then builds an Authorization: Bearer header (literally Bearer + trailing space) and an empty X-Api-Key. The HTTP layer (h11, via httpx) validates header values at write time and rejects the trailing space, so every request fails:

h11._util.LocalProtocolError: Illegal header value b'Bearer '

surfaced as anthropic.APIConnectionError. Empty-string env vars are common in CI, containers, and wrapper shells (export VAR=), and the Claude Code CLI exports both ANTHROPIC_AUTH_TOKEN= and ANTHROPIC_API_KEY= empty — so any script there that constructs Anthropic() without an explicit key fails on every call.

Root cause

The env-credential guards use is None, so an empty string slips through every check: the env read keeps "", the "missing auth" fallback (api_key is None and auth_token is None) never fires, and the header builders emit on is not None — producing {"X-Api-Key": "", "Authorization": "Bearer "}.

Fix

Coerce an empty env credential to None at read time in both the sync and async clients, so an empty value is treated as unset and credential auto-discovery runs as expected. Explicitly-passed credentials are untouched.

api_key = os.environ.get("ANTHROPIC_API_KEY") or None
auth_token = os.environ.get("ANTHROPIC_AUTH_TOKEN") or None

Verification

Reproduced the LocalProtocolError on main before the change; after the fix auth_headers == {} and auto-discovery runs. A real key still produces the correct X-Api-Key header.

• Added regression tests for both sync and async clients (tests/test_client.py).
• pytest tests/test_client.py — 170 passed.
• ruff check / ruff format --check clean; no new pyright errors on changed lines.